1. Field of the Invention
The present invention relates to a deinking method for deinking wastepapers such as newspapers, leaflets and magazines in order to recycle them. More particularly, the present invention relates to a deinking process according to which good-quality pulp sheets reduced in sad color can be obtained.
2. Prior Arts
Wastepapers are recycled by stripping ink from the wastepapers by a deinking treatment to prepare recycled pulp and manufacturing recycled paper from the recycled pulp. The conventional deinking methods generally comprise a step of stripping ink from wastepapers and a step of discharging the stripped ink.
More specifically, the deinking method comprises as main steps:
(1) a step of pulping (disintegrating) wastepapers, PA0 (2) a step of aging, i.e., leaving the disintegrated paper as it is, PA0 (3) a step of flotation, and PA0 (4) a step of washing.
That is, in deinking treatment, ink bound to the fibers of wastepapers is physically and chemically (or biochemically) stripped therefrom to thereby separate the ink from the fibers. Thus, recycled pulp is obtained.
The foregoing ink stripping in the conventional deinking treatment is generally carried out at a high pH (pH: 10 to 9), followed by the removal of the ink under the same conditions in the flotation step.
A deinking agent for use in the ink stripping step and the ink removal step has hitherto been required to have different properties for the respective steps. Specifically, a deinking agent for use in the ink stripping step is required to improve the wettability and permeability of pulp with ink and increase the ink-stripping power while having excellent ink-dispersing properties in order to prevent redeposition. On the other hand, a deinking agent for use in the ink removal step is required to have a foaming power and foam-breaking properties in the course of washing.
It has been believed that improvements in such performances can generally be attained by lowering the surface tension and hence, increasing the hydrophilicity of ink interfaces, i.e., lowering the contact angle of the aqueous solution of a deinking agent (aqueous solution portion of slurry) with ink interfaces is a matter of necessity for improvements in performances.
From the viewpoint of ink removal, however, a deinking agent having good ink-stripping and ink-dispersing properties involves a particular problem of "sad color" because the ink is excessively dispersed such that the ink is insufficiently adsorbed on the bubbles, thus failing to provide satisfactory whiteness. This "sad color" deteriorates the appearance of printed matter prepared by using recycled paper. Thus, in order to prevent this problem, a large amount of virgin pulp must be added. In other words, the problem of "sad color" cannot well be solved with any approach from the viewpoint of an improvement in the wettability of ink interfaces or an increase in the hydrophilicity of ink interfaces as has hitherto been proposed.